The geochemistry of concretions from the Kimmeridge Clay Formation of southern and eastern England

Concretions from the Kimmeridge Clay Formation are of three types: calcareous concretions, septarian calcareous concretions and pyrite/calcite concretions and nodules, which occur within different mudstone facies. Isotopic and chemical analysis of the concretionary carbonates indicate growth in the Fe-reduction, sulphate-reduction and decarboxylation zones. The septarian concretions show a long and complex history, with early initiation of growth and development spanning several phases of burial, each often resulting in the formation of septaria. Growth apparently ceased in the transitional zone between the sulphate-reduction and the methanogenesis zones. Very early growth in the Fe-reduction zones is also seen in one sample. The non-septarian concretions began growth later within the sulphate-reduction zone and have had a simpler burial history while the pyrite/calcite concretions show carbonate cementation in the sulphate-reduction-methanogenesis transition zone. A ferroan dolomite/calcite septarian nodule with decarboxylation zone characteristics also occurs. Development of concretions appears to be indirectly controlled by the sedimentation rate and depositional environment, the latter determining the organic matter input to the sediments. Calcareous concretions predominate in swell areas and during periods of low sedimentation rate in the basins with poor organic matter preservation and deposition of calcareous mudstones. Pyrite/calcite concretions occur in organic-rich mudstones deposited under higher sedimentation rates in the basins, while the ferroan dolomite nodule grew under very high sedimentation rates.